Since their introduction in the early 1980s (as illustrated by ‘High-performance heat sinking for VLSI’, Tuckerman&Pease, 5, 1981, IEEE Electron Device Letters, Vol. 2, p. 216), microchannel heat sinks have attracted much attention for cooling in high heat-flux applications, like e.g. integrated circuits. Conventional microchannel coolers are based on a parallel channels arrangement. The coolant flows from an inlet port, situated at one extreme of the chip to an outlet port situated at the other opposite extreme. In such an arrangement, cooling is more efficient near the inlet than near the outlet and a temperature gradient is formed. Furthermore the fluid flow must be large to remove a sizeable amount of heat and the channels must be narrow to optimize heat exchange between the fluid and the chip. As a consequence, the pressure necessary to operate the cooler is typically large and difficult to be provided by a compact pump. Various possibilities for decreasing the operating pressure and/or increasing the thermal exchange have been proposed, but none of them really solves the problem. It has also to be noted that a cooling solution based on parallel channels is highly inefficient in the cases where heat load varies with position (e.g. presence of hot spots).
The volume of cooling flow can be decreased if a two phase flow is used, i.e. if a liquid which evaporates at the chip temperature is used. The evaporation specific heat is much larger than the heat capacity of the fluid times the typical temperature increase (50-60° C.). Therefore, a much lower volume of fluid can be used, the operating pressure becomes lower and the requirements on the pump less stringent. However, it is very difficult to control a two phase flow through microchannels. The flow tends to be unstable and different from channel to channel. No established method exists for stabilizing a two-phase flow in microchannels.
An improved cooling arrangement consists in fabricating, both for the incoming cold fluid and for the out-going warm fluid, a plurality of flow paths perpendicular to the plate to be cooled. As inlets of cold fluid and outlets of warmed fluid are uniformly distributed over the chip, the chip temperature is uniform. Various implementations of this concept exist, e.g. in the document “Hierarchically Nested Channels for Fast Squeezing Interfaces with Reduced Thermal Resistance,” Brunschwiler et al, IEEE Transactions on Components and Packaging Technologies, 30 (2), 2007, pp.226-234, or in patent publications US-A-2006207746 and U.S. Pat. No. 6,538,885. The operation of these ‘impingement’ coolers typically requires a much smaller pressure than microchannels, making the requirements on the pump less stringent. As in the case of straight channels, the operation in a two-phase flow is not obvious and it is not possible to cope with the hot spots.
A third issue in liquid cooling of IC's is the need of a pump, which has to provide the appropriate pressure and flow. Ideally the pump should be a micropump, integrated in the cooling chip and occupying a minimum volume. As the needed pressure and flow are both large, the implementation of such a miniaturized pump is difficult.
The principle of electrowetting is known in the art and has been the subject of various studies, e.g. “Electrowetting-based actuation of droplets for integrated microfluidics”, M. G. Pollack et al, 2002, The royal society of chemistry, Lab Chip, Vol. 2, pp. 96-101. The latter reference discloses a so-called ‘electrowetting on dielectric’ system (EWOD), wherein a droplet is actuated between two parallel surfaces, with dielectric material on the walls of the surfaces, a grounded electrode embedded in the dielectric on one side, and a plurality of actuation electrodes embedded in the dielectric on the other side.
By applying a voltage to subsequent electrodes, a droplet moves forward within the channel. The principle has been proposed for use in a cooling system for electronic substrates, as shown in WO-A-2006/016293. In all of these known systems, the droplet moves parallel to the surface to be cooled, possibly in a plurality of microchannels, but as in the case of continuous flow a temperature gradient exists between the inlet and the outlet. Treatment of hot spots with this system is therefore still difficult. US-A-2008/0047701 describes a similar system wherein a droplet is moved by electrowetting towards a hot spot. Nevertheless, the control of the droplet movement in such a system is difficult and heat removal from a hot spot is therefore open to improvement.